1. Field
This disclosure relates to improving the purity of hydrogen gas using a secondary hydrogen purification method downstream of a primary hydrogen pressure swing adsorption unit.
2. General Background
A hydrogen generation unit (HGU) is a combination of thermo-chemical processes that convert a fuel-steam mixture into a hydrogen-rich gas mixture typically composed of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), water vapor (H2O) and other gases depending on the composition of the fuel feedstock. Typically this mixture is known as reformate. For many applications this reformate stream is then passed to a hydrogen purification unit in which 60% to 90% of the hydrogen is separated into a relatively pure hydrogen stream (99+% H2) and an off-gas stream consisting of the other species in the reformate mixture. One typical method used to purify the reformate is a pressure swing adsorption (PSA) unit, which consists of a series of bed filled with adsorbent material (typically but not limited to zeolites). As the pressurized reformate flows through the bed gaseous species adsorb on to the active surfaces. Since the H2 is the least strongly adsorbed species in the reformate stream, a pure H2 gas exits the bed. After a period of time when the adsorbent sites begin to become saturated, the feed gas is removed and the bed is depressurized forcing the adsorbed species to desorb and exit the bed as the off-gas stream. By cycling several beds through this pressurization and depressurization cycle a continuous H2 purification process is created. As the capacities of the beds are pushed to their limits with higher flow rates and faster cycle times, non-hydrogen gas species begin to contaminate the relatively pure H2 gas stream. Typically, the species of concern are the other gases in the reformate stream such as CH4, CO, and CO2.
Of the non-hydrogen species typically in the reformate feed to the PSA, H2O and CO2 are strongly adsorbed onto the surfaces of the zeolites and CO and CH4 are weakly adsorbed. As a result the relatively pure hydrogen stream exiting the PSA typically has CO and CH4 as the primary contaminates. In fuel cell and hydrogen refueling station applications the most critical of these contaminates is the CO, because is causes performance degradation of the fuel cell or metal hydride hydrogen storage units. CH4 is relatively non-reactive in the fuel cell and metal hydride materials, and therefore, does not cause performance degradation. It is beneficial to include a reactor between the PSA and the fuel cell that converts the CO back into CH4. This reactor allows the capacity of the PSA to be increased substantially without impacting performance of fuel cell units downstream.
One reaction mechanism that achieves this is known as methanation, which is the reverse of the steam reforming reactions. Typically in methanation reactors, a catalyst is used and ruthenium based catalysts have proven to be very effective, although other catalysts such as nickel, platinum, etc. can be used. For these catalysts to be effective the temperature of the catalyst must be greater than 150 C and preferably greater than 190 C.CO+3H2→CH4+H2O+Heat  E-1 PrimaryCO2+4H2→CH4+2H2O+Heat  E-2 Secondary
Methanation reactors have been integrated into hydrogen generation systems in the prior art, but typically they have been used upstream of the PSA unit to minimize the CO concentration entering the PSA unit. Typically, this upstream location is used because the reformate gas is at the appropriate temperature range to maintain the catalyst activity. One issue with this art is the secondary reaction identified above, where CO2 is also converted into CH4 since the CO2 concentration is typically 20% in comparison to CO concentrations in the 2 to 4% range. To manage the methanation reaction a tight temperature range is preferably maintained in the catalytic bed which promotes the reaction of CO but does not promote the reaction of the secondary reaction with CO2. Not only is this a source of process inefficiency, but it can also result in thermal run-away in which all the CO2 is reconverted back into CH4. Therefore a system and method are needed which alleviates these inefficiencies and precludes thermal run-away from occurring.